Backticks are your friends. This was tested on Windows:

#!/usr/bin/perl -w
# test-stdout.pl
use strict;
print "Hello, world.";

--------------------------

#!/usr/bin/perl -w
# test.pl
use strict;
my $prog_output = `perl test-stdout.pl`;
print "\nIt said [$prog_output]\n";
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Or if you like, you can use qx( ), which does the same thing as backticks.